"They [the US -Ed.] must have understood that they needed this Russian equipment for development of rocket technology, otherwise they would have to limit themselves in a series of expensive projects. It is evident that this is the reason the supply of our engines was continued," Stadnik told RIA Novosti.

He said Moscow and Washington continue the contracts which Russian rocket maker Energia (NPO Energomash) signed in January worth $1 billion with the US Orbital Sciences Corporation for the delivery of sixty RD-181 engines.

Under the contract, Russian cosmonauts will conduct flight training with their American counterparts. Russian engineers will also be involved in the rocket engine installation and testing.

The RD-181 engine was developed specifically for the US Antares rockets; it allows more cargo to be brought up to the International Space Station. Energia has cooperated with the US since 1990, delivering RD-180 engines for Atlas rockets.

According to Orbital, Russian rocket engines offer “the best combination of schedule availability, technical performance and cost parameters as compared to other possible options.”

The supply of engines was halted last April as result of US sanctions on many cooperation projects with Russia, including all space projects except the International Space Station (ISS). The Department of Defense was then prohibited from signing new or modifying existing contracts for launches using engines designed or manufactured in Russia.

Later, the Department of Defense asked Congress to allow Russian rocket engines to be used until 2022, in order to keep the country’s space program going.

In July Russia delivered two RD-181 engines to the US. They are expected to be used for next Antares flight in early 2016.